The claim that pulmonary fat embolism is caused by a trauma-induced coalescence of normally dispersed blood fat, and is enhanced by lipemia, is being investigated. Small amounts of labeled fat of high activity are being infused over a period of hours into normal animals, traumatized animals, lipemic animals, and traumatized-lipemic animals. The labeled fat is being recovered from the lungs and its activity compared. Thus far, no changes in activity have been found in these groups of animals. Conditions leading to systemic fat embolism are being studied. Damage to the lungs, prior to pulmonary embolization, or during embolization extending over a period of days, probably changes the pulmonary vascular bed in a manner which permits relatively large amounts of fat to pass through the lungs into the general circulation.